(a) Field of the Invention
The present invention relates to a thin film transistor array panel and a manufacturing method thereof.
(b) Description of the Related Art
A flat panel display, such as a liquid crystal display (“LCD”), an organic light emitting diode (“OLED”) display, or an electrophoresis display, for example, includes a plurality of pairs of field generating electrodes and an electro-optically activated layer interposed between the field generating electrodes.
One of the pairs of field generating electrodes is connected to a switching element and is supplied with an electrical signal, and the electro-optically activated layer converts the electrical signal to an optical signal to display images.
The LCD includes a liquid crystal layer as the electro-optically activated layer, and the OLED display includes an organic light emitting layer as the electro-optically activated layer.
A thin film transistor TFT having three terminals may be used in the flat panel display as the switching element, and the flat panel display may include a plurality of gate lines for transmitting scanning signals to control the TFT and a plurality of data lines for transmitting image signals to the pixel electrode.
The TFT includes a thin film made of amorphous silicon or polysilicon. The amorphous silicon may be deposited as a thin film at a low temperature, and thereby a substrate made of glass having a low melting point and a low cost may be used. However, the TFT including an amorphous silicon thin film has low field effect mobility. Therefore, the TFT including a polysilicon thin film is preferably used because the polysilicon thin film has high field effect mobility and a low leakage current and is able to perform a high frequency operation.
The polysilicon thin film is formed by depositing an amorphous silicon thin film and crystallizing the deposited amorphous silicon thin film. Here, the crystallization is performed by heat treatment at a high temperature or by laser beam irradiation, for example.
However, when crystallization is performed using the heat treatment, a gate conductor formed under the amorphous silicon thin film is affected by heat applied to the amorphous silicon thin film. In addition, when crystallization is performed using the laser beam irradiation, uniform crystallization may not be easily obtained due to reflection, for example, by the gate conductor.
Accordingly, a polysilicon TFT has a top gate structure. However the polysilicon TFT having the top gate structure has drawbacks which include difficulty in forming the top gate structure and high production costs of the top gate structure.